1. Field of the Invention
This invention relates to transaction systems, and more specifically to cryptographic protocols and other techniques for ensuring security and privacy.
2. Description of Prior Art
Reference is hereby made to the following U.S. patents by the present applicant that are included herein by reference: U.S. Pat. No. 4,759,063 "Blind signature systems"; U.S. Pat. No. 4,759,064 "Unanticipated blind signature systems"; U.S. Pat. No. 4,947,430 "Card computer moderated systems"; U.S. Pat. No. 4,949,380 "Returned-value blind signature systems"; U.S. Pat. No. 4,987,593 "One-show blind signature systems"; and U.S. Pat. No. 5,131,039 "Optionally moderated transaction systems."
Payment systems today structurally provide either substantially unlimited traceability of payments or substantial untraceability. Bank notes and checks are paper-based examples of each extreme. Most digital systems proposed to date are similarly polarized into substantially traceable and substantially untraceable.
A variety of perceived requirements are believed to suggest a need for systems that have some provisions for traceability. Examples include: blacklisting known abusers of a system; investigations related to violation of law;marking of bearer instruments given to suspected criminals; statistical analysis of aggregated consumer behavior; recovery of money in case of unanticipated loss of information; and maintenance and provision by participants in payments of comprehensive records.
On the other hand, a variety of perceived requirements are believed to suggest a need for some corresponding limitations on traceability. Examples include: preventing use of blacklisting mechanism for unauthorized blacklisting or tracing; controlling how many investigations are made and maintaining confidentiality of who is being investigated; preventing marking of money withdrawn from occurring more than to a limited extent; ensuring that statistical studies cannot determine individually identifiable data; preventing use of a recovery mechanism by parties other than the party whose data is being recovered; and allowing recipients and intermediaries in payments some control over clandestine or otherwise improper use of tracing information.